configure: add possibility to set arbitrary variable in check_for

If check_for is able to find the required prog/inc/lib, allow it to
set an arbitrary variable to 'y'. This variable is then pushed down
to the kconfig definition.

For example:
has_or_abort prog=foobar kconfig=has_foobar

If foobar is available, it yields a kconfig variable defaulting to y:
config CONFIGURE_has_foobar
bool
default y

If foobar is missing, it yields a kconfig variable defaulting to n:
config CONFIGURE_has_foobar
bool

Thus it is possible to depends on that variabel to show/hide options:
config SOME_FEATURE
bool
prompt "Some feature"
depends on CONFIGURE_has_foobar

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

Dmitry V. Levin [Tue, 3 Nov 2009 16:49:49 +0000]

Fix check for linux/netlink.h on Linux 2.6.32-rc5+

* configure.ac (AC_CHECK_HEADERS): In check for linux/netlink.h, include 

  sys/socket.h instead of linux/socket.h beforehand.

diff --git a/configure.ac b/configure.ac

--- a/configure.ac

+++ b/configure.ac

@@ -258,7 +258,7 @@ AC_CHECK_HEADERS([ \

 ], [], [])

 AC_CHECK_HEADERS([linux/icmp.h linux/in6.h linux/netlink.h linux/if_packet.h],

                  [], [], [#include <stddef.h>

-#include <linux/socket.h>])

+#include <sys/socket.h>])

 AC_CHECK_HEADERS([asm/sigcontext.h], [], [], [#include <signal.h>])

 AC_CHECK_TYPES([struct sigcontext_struct,

 		struct sigcontext],,, [#include <signal.h>])